Compressed Gas Cylinders and Equipment

There are two basic types of compressed gas cylinders. Non-refillable cylinders are designed for one-time use and should never be refilled or reused. Refillable cylinders are made of steel or aluminum and are designed for refilling and repeated use. Some cylinders have been in service for over 40 years. Most refillable cylinders have an open interior with walls of 1/4 inch steel and a reinforced neck and bottom. Because commonly used gases are highly pressurized (between 1,500 psig and 2,500 psig), cylinders must be maintained in good condition and protected from accidental damage at all times.

A compressed gas cylinder that has been accidentally knocked over and damaged at the valve can result in extensive property damage and personal injury. Never attempt to stop a damaged cylinder that is spinning in place due to the rapid release of pressurized contents! Leave the area until the contents are dispelled and the cylinder has stopped. 

Refillable Cylinders

Acetylene cylinders have a porous filler material that helps stabilize the extremely volatile gas. Acetone is used to stabilize the acetylene, but pockets of pure acetylene can develop at the valve stem if the cylinder is not kept in an upright position, or if the cylinder is dented or damaged. Acetylene is pressurized at 250 psig, but free acetylene is highly unstable at 15 psig. The maximum flow rate on acetylene cylinders is 15 psig. Always keep acetylene cylinders in an upright position! If a cylinder is found that is not in an upright position, put it in an upright position and let it sit for at least 24 hours before using or contacting the supplier for guidance.

Some gases, such as carbon dioxide, are commonly used in both liquid and gas forms. Cylinders designed for such use have a siphon, or "dip", tube. A siphon serves as a drinking straw to pull the liquid from the bottom of the cylinder when needed. The valve pulls the gas vapors from the top of the cylinder when the gas form is required.

Heat

Effects of heat can cause the cylinder to expand and contract with pressure changes. Cylinders are designed, to some degree, to handle such pressure changes, however, cylinders made of aluminum are affected by heat more than cylinders made of steel and can rupture. Some suppliers use invisible heat strips on the cylinders to identify damage from heat. Strips are invisible under normal temperatures but will turn orange or brown if exposed to more than 120 degrees F. 

Arcing Damage

Arcing damage from welding operations can result in a heat rise sufficient enough to cause a pressure explosion or the pressure relief device to activate. When this occurs on a fuel gas cylinder, such as acetylene, it can cause rapid dissociation within the cylinder resulting in greater destruction and/or injuries. This occurs most frequently on argon, carbon dioxide, helium, and inert mixture cylinders used in heliarc welding when the torch is left dangling on top of the cylinder. For this to happen, the cylinder must be part of the electrical circuit connecting the work to the welding machine. This is highly dangerous even without the arc burn and must not be permitted. Arc burns are easily recognized by a spot, or series of spots, of freshly burned paint, exposing bare metal. When caused by a stick electrode, there can be a deposit of filler metal on the cylinder where the electrode dragged. 

• Never allow the cylinder to become part of an electrical circuit. Do not place the torch or electrode holder on the cylinders for any reason.
• Arc-damaged cylinders must be removed from service and returned to the supplier, who often charges the customer for the cost of the cylinder. 

Dents

Dents can occur from impact or mishandling, which can weaken the walls of the cylinder making it more susceptible to rupture. 

Corrosion

Corrosion from moisture, salt, corrosives, and other materials can corrode cylinders, especially the bottom of the cylinder where stored on the ground. Always store cylinders in a dry location, preferably on a concrete surface.

Cylinders must contain the following information:

• DOT or ICC (prior to 1968) identification number - ex. DOT3AA2265. This identifies the cylinder material and the service pressure in pounds per square inch.
• Cylinder serial number - ex. SG12152A. The letters "SG" may precede the serial number to indicate a specialty gas cylinder. 
• Original owner of the cylinder - ex. APROINC
• Date of maintenance to indicate the original hydrostatic test (month/year).
• Current owner of the cylinder will appear on the neck ring.
• Retest markings (month, facility, year, rating, stamp). A "+" indicates the cylinder qualified for a 10 percent overfill. A star stamp on the end of the marking indicates the cylinder meets the requirements for a 10-year retest.
• CylinderTrak bar code provides a unique identifier and is used by computer systems to track cylinders through the filling process.
• Cylinder manufacturer's inspection marking, which is unique to the inspector.
• Cylinder tare weight, i.e. the weight of the cylinder plus the valve without product, preceded by the letters "TW."
• Cylinders must also identify their contents, which is typically in the form of a sticker applied near the neck of the cylinder. This is the only acceptable means of identifying non-medical-grade cylinder contents. There are only five medical-grade gases that are permitted to be identified by color. All others are not required to be color-coded, and color codes may vary from one supplier to another.

Pressure regulators reduce the high pressures of the stored gas in the cylinder to lower pressures that can be safely used in an operating system. Proper regulator selection is critical for both the safety and effectiveness of operating systems. Regulators are designed to control pressure; they do not measure or control flow unless equipped with devices such as a flow meter specifically designed for such purposes. Regulators can explode. If there is a problem, the regulator will explode frontward and/or backward. The safest place to be standing is always to the side of the regulator's face - preferably with the valve between you and the regulator - to avoid reaching in front of the regulator's face to open the valve. The face of the regulator should always be angled upward (provided a flow meter is not attached), so if there is an explosion, the adjusting screw and debris will fly away from your face.

Regulator connections to cylinder valves must be completely free of dirt, dust, oil, and grease. "Crack" the valve slowly (by opening the valve slightly and then reclosing it) before attaching the regulator in order to blow out dust and debris from the opening. Note: Cylinders containing highly toxic gases should not be "cracked." 

Regulators are attached to the cylinder, or manifold, at the inlet connection. This connection should be tested for leaks with a non-petroleum-based product. Note that many soaps contain petroleum! The connection is marked with a Compressed Gas Association (CGA) number and will be left-hand or right-hand threaded to match the nut or fitting. This prevents a piece of incompatible gas equipment from being connected to the wrong gas supply.

• Right-handed CGA fittings will have a smooth nut surface and have an even number for the second digit, for example, 540 (for oxygen).
• Left-handed CGA fittings will have a notched groove in the surface and have an odd number for the second digit, for example, 350 (for hydrogen). 

Never use damaged or defective equipment. In fact, it's best not to use a regulator that you do not know the history of - it may have been misused or repaired by an unauthorized person. Refer any problems or defects to the manufacturer for recommendations and authorized repair.

Regulator tips:

• Opening a regulator: Stand on the valve side of the cylinder at arm's length so you do not have to reach in front of the regulator's face. Turn your head away from the regulator and open the valve, turning counterclockwise, to blow out dust and debris, and then reclose the valve. 
• Changing a regulator: Close the valve and drain the regulator by backing out the adjusting screw. Disconnect the regulator, making sure not to touch the nut and gland areas. Connect the regulator to the new cylinder.
• Closing a regulator: Turn the valve clockwise to close the valve. Drain the regulator by turning (opening) the adjusting crew to release any gas. Reclose the adjusting screw. 
• Recommendation: To provide easier access and additional safety, purchase wall-mounted regulators which connect to the supply cylinder by hose. This will reduce the handling of the regulator and reduce the likelihood of damage.

If there is a weak point on a compressed gas cylinder, it is the valve stem. If the valve stem should be struck or damaged, the gases under high pressure will escape at high speeds. It's this rapid release that turns the cylinder into an unguided missile. Valves are generally made of brass, but may also be chrome-plated for medical gases, made of aluminum for disposable cylinders, or made of stainless steel for toxic, poisonous, pyrophoric, and corrosive gases. There are three types of valves:

• Pressure seal valve: The sealing mechanism for this two-piece valve stem is provided by a Teflon packing ring that makes contact with a ridge on the upper stem. The force that provides this contact is from a spring located in the handwheel. This spring provides upward force to the upper stem and pulls the stem's sealing ridge into the packing ring. Advantages of this type of valve are that they are extremely reliable, very strong, economical, and user-friendly. Disadvantages include: they are prone to leakage around the stem, the lubricated threads can contaminate high purity applications, and they are inappropriate for corrosives and ultra-high-purity gases.
• Packed valve: This type of valve seals by compressing a large ring of Teflon between the valve body and packing nut, which forces the Teflon to grip the stem. Advantages of this type of valve are that they are economical, and are easily opened and closed. However, they are not as good as diaphragm valves for particle generation and leak integrity, and they open very rapidly.
• Diaphragm valve: This valve uses a two-piece stem separated by non-perforated diaphragms. These diaphragms prevent leakage along the valve stem. The lower part of the stem is encased in a spring, which forces the stem away from the seat when the valve is opened. The upper stem is threaded into the diaphragm retainer nut. When the handwheel is rotated to the closed position, the upper stem pushes on the diaphragms, which deflects downward, forcing the lower stem against the valve seat. Advantages of this type of valve are that they provide superior leak integrity and have no threads or lubricants in the gas stream to generate particles or contaminants. This type of valve is required for most highly toxic or poisonous gases. Disadvantages (of older models) are that they are difficult to close, a wrench or other device may be necessary, there is a potential for malfunction, they are prone to corrosion, and they are prone to open when exposed to vibration and shock if not properly closed and secured. These problems have been eliminated in the majority of the newer models. 

Valve tips:

• Keep valves clean. Do not attempt to open a corroded valve; it may not reseal completely.
• Remember to remove the plastic caps from the opening before attaching a regulator.
• Washers may be required for some gases.
• Cylinders not having fixed handwheels must have keys, handles, or non-adjustable wrenches on the valve stem while they are in service.
• Acetylene valves shall not be opened more than one and a half turns.
• Valves shall be closed before moving a cylinder, when work is completed, and when the cylinder is empty.

Compressed gas cylinders shall have a pressure relief device installed to prevent the rupture of a normally pressurized cylinder when inadvertently exposed to fire at high temperatures. There are four basic types of pressure relief devices:

• Rupture disk devices: A flat disk typically made of metal that is designed to burst at a predetermined pressure to permit the release of gas. The pressure rating of the disk is typically stamped onto the face of the device. Examples of gases using this type of device include compressed air, argon, helium, nitrogen, and oxygen. 
• Fusible plug devices: A plug made of fusible metal designed to yield or melt at low temperatures (usually 165 or 212 degrees F). The temperature rating of the fusible metal is stamped onto the face of the device. An example of a gas that uses this type of device is acetylene. 
• Combination rupture disks/fusible plug devices: A rupture disk backed by a fusible plug. In the event of a fire, the fusible metal melts, and cylinder overpressure is relieved by the bursting of the disk. The burst pressure of the disk and the melting point of the plug will be marked with the ratings. Medical grade gas cylinders typically have this type of pressure relief device. 
• Pressure relief valves: A spring-loaded valve opens when the cylinder pressure exceeds the pressure setting of the spring to discharge contents. Once the cylinder pressure decreases to the valve's pressure setting, the valve will normally reseat without leakage.